Gas bearing system

ABSTRACT

A gas bearing system comprising two opposing substantially parallel bearing surfaces ( 2, 3 ) and at least one gas duct ( 6 ) for supplying gas through an orifice ( 7 ) to the bearing gap ( 5 ) between the bearing surfaces ( 2, 3 ). At least one of the bearing surfaces ( 2, 3 ) is provided with at least one cavity ( 8, 9 ) extending over 0.3 mm 2  to 3 mm 2  of the at least one bearing surface ( 2, 3 ). The content of the cavity ( 8, 9 ) may be between 0.3 mm 3  and 4 mm 3 . The gas duct ( 6 ) may supply gas to the cavity ( 8 ) through an orifice ( 7 ) in the wall of the cavity ( 8 ).

The invention is related to a gas bearing system comprising two opposingsubstantially parallel bearing surfaces and at least one gas duct forsupplying gas through an orifice to the bearing gap between said bearingsurfaces.

The gas bearing system may have substantially flat bearing surfaces, sothat it can be used to support and guide a member making a translatingmovement. Such gas bearing systems are frequently used as guiding andsupporting elements in high precision machines. The bearing surfaces mayalso have a cylindrical shape, so that a rotating member can besupported. Also other shapes—adapted to the relative movement of thebearing surfaces—are possible, for example a spherical shape to supporta member making a tumbling movement.

In general, such a gas bearing system must have a relatively highstiffness, but there must also be an effective damping of vibrations inthe bearing system, especially in case the gas bearing system is used inhigh precision machines, like coordinate measuring machines.

To achieve a high servo performance, gas bearings have to fulfil certainrequirements with respect to stiffness and damping. Especially when gasbearings are used in high precision machines, the stiffness and dampingof the gas bearing may have a significant influence on the entiredynamic performance of the machine. Often, such gas bearings areoptimized with respect to stiffness, but to achieve a high servoperformance, damping is an important issue as well.

The object of the invention is to provide an improvement of gas bearingsystems resulting in considerably higher damping, while the stiffnessremains relatively high.

In order to accomplish that objective, at least one of said bearingsurfaces is provided with at least one cavity extending over 0.3 mm² to3 mm², preferably over 0.5 mm² to 2 mm², of said at least one bearingsurface. In one preferred embodiment the content of said cavity isbetween 0.3 mm³ and 4 mm³, preferably between 0.5 mm³ and 2 mm³, andmore preferably between 0.7 mm³ and 1.5 mm³. One or more cavities may bepresent in one of the bearing surfaces, but also both bearing surfacesmay be provided with such cavities.

In practice it has been found that such a cavity, and especially more ofsuch cavities, have only little influence on the stiffness of thebearing, so that the stiffness remains high, but the presence of suchcavities increases the capacity of damping vibrations in the systemconsiderably.

In one preferred embodiment said gas duct supplies gas to said cavitythrough an orifice in the wall of said cavity. In case there is morethan one gas supply duct, each of the gas supply ducts may be providedwith a cavity at its mouth, and apart from that more cavities may bepresent in the bearing surfaces.

A gas bearing system provided with a cavity at the mouth of the gassupply duct is disclosed in U.S. Pat. No. 4,844,194, wherein a number ofcavities are present in one of the bearing surfaces, and an orifice forsupplying gas to the bearing gap between the substantially parallelbearing surfaces is present in the wall of each cavity. The dimensionsof the cavities are very small, but the diameter parallel to saidbearing surfaces is large to avoid a throttle effect between the mouthof the orifice and the opposing bearing surface. Without the smallcavity there would be a relatively strong and varying throttle effect atthe mouth of the orifice, caused by the small and varying distancebetween the two bearing surfaces. The distance between the twosubstantially parallel bearing surfaces is for example between 0.005 mmand 0.01 mm, and the diameter of the orifice is for example between 0.1mm and 0.2 mm.

Preferably, the depth of the cavity is at least two times, preferably atleast four times, more preferably at least six times the diameter ofsaid orifice, said orifice having a diameter between 0.05 mm and 0.3min, preferably between 0.07 mm and 0.25 mm, and more preferably between0.1 mm and 0.2 mm.

In a preferred embodiment the dimension of the cavity in a directionparallel to said bearing surface is between 0.5 mm and 2.5 mm,preferably between 0.5 mm and 1.5 mm, more preferably between 0.7 mm and1.2 mm. The cavity may have a substantially cylindrical shape, with theaxis of the cylinder being directed substantially perpendicularly tosaid substantially parallel bearing surfaces. Such a cavity can beeasily manufactured by a material removing drilling operation.

Preferably, the depth of the cavity is between 0.3 mm and 2 mm, morepreferably between 0.5 mm and 1.5 mm, and in a more preferred embodimentbetween 0.7 mm and 1.2 mm.

Very good results have been achieved in a gas bearing system comprisinga square flat bearing surface with sides of 40 mm, with one of thebearing surfaces being provided with four cylindrical cavities having adiameter of 1.5 mm and a depth of 1 mm, and the gas supply ductdebouching into one of the cavities, the diameter of the orifice being0.15 mm.

The invention furthermore relates to a high precision machine comprisinga gas bearing system as described above.

The invention will now be explained in more detail hereinafter by meansof a description of different embodiments of a gas bearing systemprovided with flat bearing surfaces, in which reference is made to adrawing, in which:

FIG. 1 is a sectional view in perspective of a prior art gas bearingsystem;

FIG. 2 is a similar view of a first embodiment of a gas bearing systemaccording to the invention;

FIG. 3 is a similar view of a second embodiment of a gas bearing systemaccording to the invention; and

FIG. 4 shows again a prior art gas bearing system.

The figures are merely schematic representations of the embodiments, inwhich some dimensions are out of proportion to achieve a betterrepresentation of relevant details.

FIG. 1 shows a gas bearing system comprising a first bearing member 1having a bearing surface 2, opposing a second bearing surface 3 of asecond bearing member 4. According to this embodiment both bearingsurfaces 2,3 are flat planes and the bearing surfaces 2,3 are directedparallel to each other. The bearing members 1,4 may be made from metalor plastic or another material.

Between the two bearing surfaces 2,3 is a bearing gap 5 into which air,or another gas, is brought through air supply duct 6 in bearing member1. Air supply duct 6 terminates near bearing surface 2 and is connectedwith bearing gap 5 by an orifice 7 restricting the airflow.

The first bearing member 1 can be present at a fixed location in amachine, while the second bearing surface 3 of the second bearing member4 can move over the fixed first bearing surface 2 to guide and supportanother part of the machine. Bearing member 4 is supported by an aircushion in the bearing gap 5 between the two bearing surfaces 2,3. Morethan one orifice 7 can be present to supply air to the bearing gap 5between the two bearing surfaces 2,3 to maintain the air cushion. It isalso possible to provide the moving bearing member 4 with an air supplyduct instead of the air supply duct 6 in bearing member 1, or additionalto air supply duct 6.

The dimensions of the bearing system can be as follows. The bearingsurfaces 2,3 may have a dimension of about 20 cm². The distance betweenthe two bearing surfaces 2,3 can be between 0.005 mm and 0.01 mm. Thediameter of the orifice 7 can be between 0.1 mm and 0.2 mm, and itslength is for example 1 mm.

FIG. 2 shows a first embodiment of a gas bearing system according to theinvention. The bearing surface 2 is provided with one cavity 8 and withtwo cavities 9. All three cavities 8,9 have a cylindrical shape with adiameter of 1.5 mm and a length (or depth) of 1 mm. The axis of thecylinder is directed perpendicularly to the bearing surface 2.

Each cavity 9 is closed at all sides, except for the side facing thebearing gap 5. Air supply duct 6 is connected with cavity 8 by orifice 7in the bottom of cavity 8, so that the supplied air flows through thecavity 8 before it reaches the bearing gap 5 between the two bearingsurfaces 2,3.

It has been found that the presence of one or more cavities 8,9 in thebearing surface 2,3 results in a considerable increase of the capacityof damping vibrations in the gas bearing system, i.e. vibrations ofbearing member 4 relative to bearing member 1, while the stiffness ofthe bearing system remains high.

FIG. 3 shows a second embodiment of the gas bearing system according tothe invention, wherein similar parts are indicated by means of the samereference numbers as in FIG. 2. In this embodiment the axis 12 of theair supply duct 6 is positioned parallel to the bearing surface 2. Thereare a number of orifices 7 between the air supply duct 6 and the bearinggap 5 between the two bearing surfaces 2,3; only two of such orificesare shown in FIG. 3. The cavities 8 as well as the orifices 7 aremanufactured by a material-removing drilling operation.

An additional advantage of the use of the cavities 8 in the gas bearingsystem as shown in FIG. 3 is the presence of more material between theair supply duct 6 and the bearing surface 2. The thickness of thatmaterial equals the length of the orifice 7 plus the depth of the cavity8. This increased thickness facilitates the manufacturing of bearingmember 1 of the gas bearing system. The dimension 10 of that material,i.e. said thickness, is indicated in FIG. 3.

FIG. 4 shows a similar gas bearing system according to the prior art,wherein dimension 11 is also the distance between air supply duct 6 andbearing surface 2. Because the length of orifice 7 is limited, saiddimension 11 is much smaller than dimension 10 in FIG. 3, so that thereis only a very small thickness of the material between air supply duct 6and bearing surface 2.

The embodiments as described above are merely examples; a great manyother embodiments are possible, for example gas bearing systems havingcylindrical bearing surfaces, where one of the bearing members rotatesaround the axis of the cylinder and cavities are present in at least oneof the bearing surfaces for damping vibrations in the system. Also othershapes—adapted to the relative movement of the bearing surfaces 2,3—arepossible, for example a spherical shape to support a bearing member 4making a tumbling movement.

1. A gas bearing system comprising two opposing substantially parallelbearing surfaces (2,3) and at least one gas duct (6) for supplying gasthrough an orifice (7) to the bearing gap (5) between said bearingsurfaces (2,3), characterized in that at least one of said bearingsurfaces (2) is provided with at least one cavity (8,9) extending over0.3 mm² to 3 mm² of said at least one bearing surface (2) and that thecontent of said cavity (8,9) is between 0.3 mm³ and 4 mm³.
 2. A gasbearing system as claimed in claim 1, characterized in that said gasduct (6) supplies gas to said cavity (8) through an orifice (7) in awall of said cavity (8).
 3. A gas bearing system as claimed in claim 1,characterized in that the depth of the cavity (8,9) is at least twotimes the diameter of said orifice (7).
 4. A gas bearing system asclaimed in claim 1, characterized in that said orifice (7) has adiameter between 0.05 mm and 0.3 mm.
 5. A gas bearing system as claimedin claim 1, characterized in that the dimension of the cavity (8,9) in adirection parallel to said bearing surface (2) is between 0.5 mm and 2.5mm.
 6. A gas bearing system as claimed in claim 1, characterized in thatthe cavity (8,9) has a substantially cylindrical shape, the cylindricalaxis being directed substantially perpendicularly to said substantiallyparallel bearing surfaces (2,3).
 7. A gas bearing system as claimed inclaim 1 characterized in that the depth of the cavity (8,9) is between0.3 mm and 2 mm.
 8. A machine comprising a gas bearing system,characterized by a gas bearing system as claimed in claim
 1. 9. A gasbearing system comprising two opposing substantially parallel bearingsurfaces (2,3) and at least one gas duct (6) for supplying gas throughan orifice (7) to the bearing gap (5) between said bearing surfaces(2,3), characterized in that at least one of said bearing surfaces (2)is provided with at least one cavity (8,9) extending over 0.3 mm² to 3mm² of said at least one bearing surface (2) and in that the depth ofthe cavity (8,9) is at least two times the diameter of said orifice (7).10. A gas bearing system as claimed in claim 9, characterized in thatthe content of said cavity (8,9) is between 0.3 mm³ and 4 mm³.
 11. A gasbearing system as claimed in claim 9, characterized in that said gasduct (6) supplies gas to said cavity (8) through an orifice (7) in awall of said cavity (8).
 12. A gas bearing system as claimed in claim 9,characterized in that said orifice (7) has a diameter between 0.05 mmand 0.3 mm.
 13. A gas bearing system as claimed in claim 9,characterized in that the cavity (8,9) has a substantially cylindricalshape, the cylindrical axis being directed substantially perpendicularlyto said substantially parallel bearing surfaces (2,3).
 14. A machinecomprising a gas bearing system, characterized by a gas bearing systemas claimed in claim
 9. 15. A gas bearing system comprising two opposingsubstantially parallel bearing surfaces (2,3) and at least one gas duct(6) for supplying gas through an orifice (7) to the bearing gap (5)between said bearing surfaces (2,3), characterized in that at least oneof said bearing surfaces (2) is provided with at least one cavity (8,9)extending over 0.3 mm² to 3 mm² of said at least one bearing surface (2)and in that the depth of the cavity (8,9) is between 0.3 mm and 2 mm.16. A gas bearing system as claimed in claim 15, characterized in thatthe content of said cavity (8,9) is between 0.3 mm³ and 4 mm³.
 17. A gasbearing system as claimed in claim 15, characterized in that said gasduct (6) supplies gas to said cavity (8) through an orifice (7) in awall of said cavity (8).
 18. A gas bearing system as claimed in claims15, characterized in that the dimension of the cavity (8,9) in adirection parallel to said bearing surface (2) is between 0.5 mm and 2.5mm.
 19. A gas bearing system as claimed in claims 15, characterized inthat the cavity (8,9) has a substantially cylindrical shape, thecylindrical axis being directed substantially perpendicularly to saidsubstantially parallel bearing surfaces (2,3).
 20. A machine comprisinga gas bearing system, characterized by a gas bearing system as claimedin claim 15.